This article is a critical feminist analysis of the UK Gender Recognition Act of 2004. This Act is radical in enabling transgenders to gain certificates recognising their new ‘acquired gender’ without undergoing hormonal or surgical treatment. The Act has considerable implications for marriage, for motherhood and fatherhood, for women who are the partners of men or women who ‘transition’ and for ‘women-only’ spaces. It is based on confusing and contradictory notions of the difference between sex and gender. As such it should be of great interest to feminists but there has been a dearth of feminist commentary. The understandings of sex and gender and of the importance of the Act will be explored here through analysis of the parliamentary debates and public responses.